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Fair value of financial instruments - Differences between Fair Value and Principal Balance for Loans Held for Sale Measured at Fair Value (Details) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
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Aggregate fair value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Mortgage loans held for sale measured at fair value | $ 683,770 | $ 262,518
Past due loans of 90 days or more | 83 | 0
Nonaccrual loans | 6,406 | 0
Aggregate fair value | Commercial |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Mortgage loans held for sale measured at fair value | 208,914 |
Aggregate Unpaid Principal Balance |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Mortgage loans held for sale measured at fair value | 651,887 | 254,868
Past due loans of 90 days or more | 163 | 0
Nonaccrual loans | 12,033 | 0
Aggregate Unpaid Principal Balance | Commercial |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Mortgage loans held for sale measured at fair value | 226,867 |
Difference |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Mortgage loans held for sale measured at fair value | 31,883 | 7,650
Past due loans of 90 days or more | (80) | 0
Nonaccrual loans | (5,627) | $ 0
Difference | Commercial |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Mortgage loans held for sale measured at fair value | $ (17,953) |

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